This invention relates generally to an engine control system and, more particularly, to a method and apparatus for delivering multiple fuel injections to the cylinder of an engine during a fuel injection event wherein the pilot shot is delivered during the intake stroke to improve fuel atomization and control exhaust emissions.
Electronically controlled fuel injectors are well known in the art including both hydraulically actuated electronically controlled fuel injectors as well as mechanically actuated electronically controlled fuel injectors. Electronically controlled fuel injectors typically inject fuel into a specific engine cylinder as a function of an injection signal received from an electronic controller. These signals include waveforms that are indicative of a desired injection rate as well as the desired timing and quantity of fuel to be injected into the cylinders.
Emission regulations pertaining to engine exhaust emissions are becoming more restrictive throughout the world including, for example, restrictions on the emission of hydrocarbons (HC), carbon monoxide, the release of particulates, and the release of nitrogen oxides (NOx). Tailoring the number of injections and the injection rate of fuel to a combustion chamber, as well as the quantity and timing of such fuel injections, is one way in which to control emissions and meet such emission standards. As a result, multiple fuel injection techniques have been utilized to modify the burn characteristics of the combustion process in an attempt to reduce emission and noise levels. Multiple injections typically involve splitting the total fuel quantity to the cylinder during a particular injection event into a plurality of fuel injections such as two separate fuel injections, such as a pilot injection and a main injection. At different engine operating conditions, it may be necessary to use different injection strategies in order to achieve both desired engine operation and emissions control. As used throughout this disclosure, an injection event is defined as the injections that occur in a cylinder during one cycle of the engine. For example, one cycle of a four cycle engine for a particular cylinder, includes an intake, compression, expansion, and exhaust stroke. Therefore, the injection event in a four stroke engine includes the number of injections, or shots, that occur in a cylinder during the four strokes of the piston. The term shot as used in the art may also refer to the actual fuel injection or to the command current signal to a fuel injector or other fuel actuation device indicative of an injection or delivery of fuel to the engine.
The timing of the various fuel injections, the pressure and quantity of fuel associated with each fuel injection or fuel shot, the number of injections, and the time delay between each fuel injection or fuel shot will control the type of combustion achieved and the resulting exhaust emissions based upon the particular operating conditions of the engine. For example, if the pilot fuel does not sufficiently atomize before combustion or light off, or if the fuel coats the cylinder wall, a non-uniform fireball or flame front may occur during ignition causing incomplete combustion. This results in hydrocarbons and other emission constituents being exhausted to the atmosphere. The same can likewise be true if the multiple fuel injections occur too close together within a given fuel injection event, or if the timing sequence between the multiple injections is sufficiently large as to likewise result in poor air/fuel mixing, coating the cylinder walls with fuel, allowing fuel to be injected beyond a desired stopping point, and/or other factors resulting in incomplete combustion. In addition, if two fuel injections are closely coupled in time to each other, rate shaping effects will produce a fuel quantity delivered which may be different from the desired fuel quantity. All of these situations can adversely affect exhaust emissions and fuel economy.
In the past, the controllability of split injections has been somewhat restricted by mechanical and other limitations associated with the particular types of injectors utilized. In addition, in some embodiments, such as disclosed in the patent U.S. Pat. No. 5,740,775, the total fuel quantity associated with a split injection is apportioned such that approximately 50% of the fuel is associated with the first fuel shot and approximately 50% of the fuel is associated with the second fuel shot. Under the more restrictive emissions regulations of today, this fuel partitioning strategy yields higher than desirable hydrocarbons and excessive fuel dilution of the oil, even with the first injection is advanced into the early portion of the compression stroke, or even into the intake stroke. Even with more advanced electronically controlled injectors, during certain engine operating conditions, it is sometimes difficult to accurately control fuel delivery, even when utilizing current control signals.
It is therefore desirable to control the delivery of the multiple fuel injections during a single fuel injection event so as to achieve better fuel atomization, better emissions control, and better fuel economy.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention there is disclosed an electronically controlled fuel injection system which is capable of delivering multiple fuel injections to a particular cylinder of an internal combustion engine during a single injection event such as three of more fuel shots generally referred to as first or pilot shot, a second or main shot, a third or anchor shot, and a post ignition shot. When delivering the first or pilot fuel shot, the present system delivers the pilot shot during the intake stroke and, preferably, when the cylinder piston is located at or near the start of the intake stroke when the piston is high in the cylinder and the cylinder pressure is relatively low. Depending upon when the pilot fuel shot is injected or delivered during the intake stroke, this injection timing will prevent and/or at least reduce the possibility that the fuel from the pilot shot will directly hit the cylinder wall and such injection timing will provide better mixing and atomization of the fuel with the air in the cylinder as the piston moves through both the intake and compression strokes. In addition, a more uniform fireball or flame front is achieved when the pilot shot is ignited during the compression stroke due to improved fuel atomization and other factors. The main and/or anchor fuel injections are then delivered during the compression stroke, or shortly thereafter, into the uniform flame front at a slightly higher pressure as compared to delivery of the pilot fuel injection. In this regard, the second or main fuel injection shot as well as a third or anchor fuel injection shot will preferably take place when the cylinder position is near top dead center during the compression stroke. In some situations, the main and/or anchor fuel shots may occur or continue to occur when the cylinder position is near top dead center during the power stroke. The present system also includes means for varying the timing and fuel quantity associated with each fuel injection shot as well as the time interval between the various fuel injection shots based upon the particular operating conditions of the engine and based upon the desired results.
These and other aspects and advantages of the present invention will become apparent upon reading the detailed description in connection with the drawings and appended claims.